National Oceanic and Atmospheric Administraton National Marine Fisheries Service Northwest Fisheries Science Center 2725 Montlake Blvd. E. Seattle, WA 98112 Fish and Fish Oil in the Diet and its Effects on Certain Medical Conditions A general discussion largely in non-technical language Maurice E. Stansby Scientific Consultant July 1991
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National Oceanic and Atmospheric Administraton
National Marine
Fisheries Service
Northwest Fisheries Science Center 2725 Montlake Blvd. E. Seattle, WA 98112
Fish and Fish Oil
in the Diet
and its Effects on
Certain Medical
Conditions A general discussion largely in non-technical language
Maurice E. Stansby Scientific Consultant
July 1991
Fish and Fish Oil in the Diet and its
Effects on Certain Medical Conditions
A General Discussion Largely in Non-technical Language
Maurice E. Stansby Scientific Consultant
National Oceanic and Atmospheric Administration National Marine Fisheries Service Northwest Fisheries Science Center 2725 Montlake Blvd. East Seattle, Washington 98112
(.
l ·.,. : .
Chapter 1 - Introduction
In June 1990 a book, "Fish Oils in Nutrition" which I had
edited was published. This book was written for scientists and
was accordingly, for the most part, not readily understandable to
readers with limited or no background in such topics as
chemistry, nutrition or the like. In 1985 I had published a
report, "Medical Effects of Fish or Fish Oil in the Diet."
Although this report was written employing limited scientific
wording, it would still be to some extent difficult for the
illill\ average reader without scientific background to understand.
This present report has been assembled to bring up-to-date
much of the material given in the 1985 report, yet on the whole,
eliminating most of the more scientific wording so that it could
be readily understandable to any readers. In some instances the
text will make reference to the Appendix where some of the more
scientific aspects have been enlarged upon.
The main portions of this article will not have references
in the text listed at the end of each chapter. This is being
done because, for the most part, the references are nearly all
written from the standpoint that the reader can readily
understand scientific terminology.
1
I
This article will have 7 sections (chapters). In addition
to this Chapter 1, Introduction, the following chapters are
included. Chapter 2 entitled "Chemical Aspects of Fish Oils"
covers in simple language information on such aspects as the
make-up of fish oils, the variation of the amount of fatty acids
in fish oils made from the same species etc. Chapter 3 on
nutritional effects of fish oils for humans brings what is known
together covering both information known long ago up to recent
findings.
The next section, Chapter 4, describes how the fatty acids
of fish oils which result in beneficial medical effects also 1'\._,
render them highly vulnerable to oxidation and how this effects I
the oils. Chapter 5 deals with the manufacture of crude fish
oils followed by processing to give refined oils which are
suitable for human consumption. Chapter 6 relates to compounds
sometimes found in fish oils other than triglycerides and
phospholipids. Most of it relates to glyceryl ethers.
The last section, Chapter 7, designated as Appendix contains
material with somewhat more scientific terminology for use by
readers who can understand it. For example, in Chapter 2 there
are listed many compounds with their odor or flavor which occur
sometimes in fish oils. In the text in Chapter 2, the chemical
names for these substances are not given; rather they are listed
as substance #1, 2, 3, etc. For those readers who might be
2
interested in just which chemical compounds are involved the
scientific name such as 1, 3, 5 trimethyl benzene or isobutanoic
acid are given in the Appendix for each of the various substances
which in the text of Chapter 2 had been listed only as numbers.
3
Chapter 2. Chemical Aspects of Fish Oils
I. Composition of Foods
Most foods contain water, protein sometimes carbohydrates,
fat or oil, ash (minerals) and sometimes vitamins. Fish
ordinarily contain water, protein, carbohydrates, fat or oil,
minerals, and vitamins (see Appendix, page 86). The amount of
carbohydrates in fish is quite small except in shellfish and
crustaceans such as clams and shrimp where their amount often can
be considerable sometimes up to a range of 3 to 4 percent. The
amount of vitamins in percent is always extremely small yet from
a nutritional standpoint it may be quite significant.
Usually the sum of the amount of oil and water in fish is
equal to close to 80%. -Thus, if the oil content is 10% the water
content is about 70% or if the oil content is 1% the water
content is usually about 79%. The components of fish which can
give them the ability to decrease incidence of certain diseases
such as those of the heart occur in the oil of the fish.
II. Oil or Fat in Fish
Whereas the fat in such foods as meat or poultry occurs as
solid fat, that in fish is liquid at room temperature and is,
therefore an oil. Hereafter we will use the term fish oil rather
4
than fat. The amount of oil in the edible portion of most
species of fish can vary tremendously both from species to
species and also from one sample of a species to another sample
of the same species. Because of this high variation, before the
chemical nature of the oil of fish is discussed, a detailed
section is included here to describe this variation. Because of
this wide variation, the content of oil in a sample of fish is
ordinarily of greater importance from a nutritional standpoint
than is the chemical nature of the oil.
III. Amount of Oil in Edible Portions of Fish
In samples of fish from different species, the oil content
in edible portions very often varies from 0. 3% to 14%, a
difference of 35 times. This is a very conservative estimate.
Actually in quite a few instances the variation can be greater.
For example, it is not unusual for some lean species of fish like
cod to have as little as 0. 2% oil while fatter fish such as
mackerel can, at peak of production, have as much as 20% oil.
This is a difference of 100 times. In a few rare instances, for
example, with siscowet trout caught in deep areas of Lake
Superior the oil content of the edible flesh has been found to be
as high as 70%. For very lean fish occasionally the oil content
of the edible portion can be as low as 0. 2%. This is a variation
of 350 times.
5
The oil content of fish which undergo long spawning migra
tions vary with the period of the migration. The fish initially
build up fairly high oil content in their flesh. When they begin
a long spawning migration, they may not eat any food. Instead
they use a portion of their excess body fat which occurs in the
flesh. For example, pink salmon, which may begin the season with
an oil content up to 11% or a little more, use up their fat as a
source of calories during their spawning migration at the end of
which their fat content may be only 3% or less. In the salmon
canning industry, the fish are caught and canned ordinarily
before any huge decline in oil content has taken place.
Table 2-1 shows average oil content and range of oil
contents in the edible flesh of a number of common species of
fish.
6
Table 2-1. Average and Ranges of Oil Content'l of 17 Common
Species of Fish.
Species
Carp
Chub, lake
Cod, Atlantic
Flounder
Haddock
Mackerel, Atlantic
Ocean perch, Pacific
Perch, yellow
Pike, yellow
Rockfish (Sebastes)
Salmon, Chum
Salmon, Coho
Salmon, Pink
Salmon, Sockeye
Smelt, Marine
Smelt, Lake
Whitefish, Lake
% of
Range
Oil Content
1. 0-12. 0
4. 0-13. 0
0. 2-0. 9
0. 3-3. 4
0. 2-0. 6
2. 7-25
3. 0-6. 0
0. 8-1. 2
0. 8-3. 0
1. 2-4. 3
2. 2-7. 3
3. 3-11. 2
3. 2-11.6
7. 8-13. 7
4. 6-8. 8
1. 5-3. 3
4. 7-18. 8
Average
% of Oil Content'1 v
5. 0
8. 5
0. 4
1. 4
0. 35
14. 0
4. 2
0. 9
1. 3
2. 5
4. 0
7. 0
6. 5
11. 3
6. 3
2. 4
9. 6
'l Average value based upon fish taken during the main
industrial harvest season, although even then a fairly large
range of fat content usually occurs.
7
The oil content of different parts of the edible flesh vary
as shown in Table 2-2. Ordinarily the flesh taken from near the
head has a higher oil content than is the case with that taken
from near the tail.
Table 2-2. Variation in the Oil Content of Different Portions of
the Edible Flesh of the Same Fish.
Species Section Analyzed Oil %
Red King Salmon Near Head 20. 2
Red King Salmon Near Tail 11.1
White King Salmon Near Head 15.1
White King Salmon Near Tail 8.1
Yellowfin Tuna Middle 3. 2
Yellowfin Tuna Near Tail 1. 4
From what has been said, it is now obvious that the oil
content of fish can vary over extremely wide ranges and that the
amount present is not determined solely by the particular species
of fish involved.
IV. Chemical Nature of Fatty Acids in Fish Oils - see also
Appendix (Chapter 7, page 87).
Most of the oil in fish consists of triglycerides where
three molecules (often 3 different molecules) of fatty acids are
grouped together and are called triglycerides. A much smaller
8
proportion of the oil of most fish are attached to a molecule
called phospholipids (and which contains phosphorous).
The fatty acids in any oil or fat are of two varieties,
saturated or unsaturated. The fatty acids consist of a long
chain of carbon atoms (designated as C) thus
Each carbon
and the last
H H H 0 HC-C-C . . . . . . . etc - c�
H H H OH
atom has attached to it hydrogen
carbon atoms has � the group c.:._ - - OH
atoms (H) as shown
which makes it
into an acid rather than some other compound. The saturated
fatty acids have each carbon atom in the long chain attached to
an adjacent carbon atom by a single bond. Two other types of
fatty acids are the monoenes in which just two of the carbon
atoms are connected by two rather than one bond thus C=C. This
is known as a double bond. The third type of fatty acid contains
two or more pairs of fatty acids connected by double bonds.
These are known as polyunsaturated fatty acids. Vegetable oils
contain mostly two or, at the most, three such double bonds
within a molecule except that in a few cases a few tenths of 1%
of fatty acids with four double bonds can occur. Fish oils, on
the other hand, often contain five or six such double bonds
within one molecule. Fish oils also, of course, contain
saturated and monoene fatty acids.
9
Fatty acids vary in their nature in another way. This has
to do with the location of the double bonds within the molecule.
Ordinarily in counting the carbon atoms in an organic compound
where these carbon atoms are arranged in a long chain, the number
1 carbon atom is the one where the group characteristic for the
compound is located. In an acid this characteristic carbon atom
would be the one making it an acid, i. e. the� group. OH
However when the nutritional properties of a fatty acid are
involved, the carbon atoms are counted from the other, far end of
the carbon chain. This far end is designated as the omega end
(omega is the last letter in the Greek alphabet).
An omega-6 fatty acid would have the following arrangement: H H H H H H H � HC-C-C-C-C-C=C .. . . ... -c, and an omega-3 fatty acid as follows: H H H H H H H OH
H H H � HC-C-C=C . .. .. .. -� Another system for naming such fatty H H H OH
acids is to substitute "n-" for omega. Thus, an omega-3 acid can
be called a n-3 acid.
Fatty acids are generally indicated by a C followed by the
number of carbon atoms present; then a colon is followed by the
number of double bonds present; if known, this is then followed
by Q)-3,ro-6, or whatever. Thus C20:Sro-3 indicates a compound with
20 carbon atoms, 5 double bonds of the omega-3 type. It could
also be designated as C20:Sn-3.
10
V. Fatty Acid Content of Oils of Different Fish Species
Unlike the oil content of different fish species about which
a great deal is known, very little is known about the fatty acid
content of different species. This is not to say that there is
little in the literature on this matter. The problem is that
most investigators, unaware that there can be a considerable
variation in fatty acid content of the same species taken at
different places and at different times, have used totally
inadequate number of fish samples to give any idea as to the
range of values representative for the species. Thus the
literature is full of many totally inaccurate values which are
for the most part quite meaningless.
This situation has arisen because back in the late
nineteenth century there was the belief that the oil of any given
species of fish always had identical properties. In those days
fatty acids were not measured. Rather various factors such as
iodine numbers were employed. The iodine number is actually
based upon the proportion of fatty acids relating to the degrees
of saturation or unsaturation. In this early time period the
iodine number would be determined of a sample of oil by some well
established laboratory. It was determined to a high degree of
accuracy and might be reported as 140. 07. Actually today we know
that iodine numbers of samples of oil of the same species can
vary considerably. Thus, the oil with the iodine number 140. 0 7
11
might vary from one sample to another by something like 130-155.
In the 19th century if some other investigator used an oil of the
species for which the value 140.07 had been determined, but they
found some other value, it was assumed that this was due to inept
analysis by the individual carrying out the test.
These erroneous ideas about the chemical make-up of fish
oils of the same species carried over into the twentieth century.
Thus often after it became customary to carry out fatty acid
analyses, it was still believed that there was little or no
difference in such results for any or all oil samples of a given
species. Most initial analyses, therefore, were made on the oil
using only a few fish usually all taken from the same location
and at the same time. The current literature is full of such
erroneous values for the fatty acid make-up of fish oils.
Actually there are only 3 fish oils for which there are
sufficient fatty acid values available which are reasonably
accurate. These are commercial menhaden oil, commercial herring
oil and the oil of the food fish, mullet. For commercial
menhaden oil, very good analytical results are available for
samples of oil taken in quite a few locations on the Gulf and
Atlantic Coasts and at different times over an 11 year period.
The results of these analyses are shown in Table 2-3. The
samples used for analysis in this table are each from large
commercial tanks, each representative of millions of fish.
12
Table 2-3. Range of Fatty Acid Content in Large Batches of
Commercial Menhaden Oil Taken Annually from 1977 Through 1988.
Range of Total Ratio
Fatty Acid Fatty Acids Highest to Lowest
Cl4:0 7. 2-12. 1 1. 7
ClS:O 0. 4-2.3 5.8
Cl6:0 15. 3-25. 6 1. 7
C16:1 9. 3-15. 8 1. 7
Cl6:2 0. 3-2. 8 9. 4
Cl6:3 0. 9-3. 5 3. 9
Cl6:4 0. 5-2. 8 5. 6
Cl7:0 0. 2-3. 0 15. 0
Cl8:0 2.5-4. 1 1. 6
Cl8:1 8. 3-13. 8 1.7
Cl8:2 0. 7-2. 8 4. 0
C18:3 0. 8-2. 3 2. 9
Cl8:4 1. 7-4. 0 2. 4
C20:0 0. 1-0. 6 6. 0
C20:4 1. 5-2. 7 1. 8
C20:5 11. 1-16. 3 1. 5
C22:1 0. 1-1. 4 10. 0
C22:5 1. 3-3. 8 2. 9
C22:6 4. 6-13. 8 3. 0
Source: Zapata-Haynie Co. , Anthony Bimbo, laboratory director
13
It should be noted that there is a very large difference in
results for each fatty acid as shown in the last column. Of the
20 fatty acids for which results are shown only 3 had a
difference in values as little as 50%. Two fatty acids (C17:0
and C22:1 had variability of from 10 to 15 times. There was a
variation of 2 1/2 times or more for 11 of the fatty acids
listed. These results demonstrate quite clearly how inaccurate
results can be. Most of the data in the literature are based
upon so few samples that the results give no idea as to the
actual range of fatty acids for the particular species.
Lest it be thought that these results are something that
relate only to commercial menhaden oil, very similar variation
occurs for commercial herring oil for which considerable data are
available. Table 2. 4 gives similar variation and the values are
for herring caught on both the Atlantic and Pacific Coasts of
North America.
14
Table 2-4. Range of Fatty Acid Composition of Commercial North
American Herring Oils.
Range of Total
Fatty Acid Fatty Acids
Cl4:0 4. 6-8. 4
Cl6:0 10. 1-18. 6
Cl6:1 6.2-12. 0
Cl8:0 0. 7-2.1
Cl8:1 9. 3-25. 2
C18:2 0.1-0. 6
Cl8:3 0.0-1. 1
Cl8:4 1. 1-2. 8
C20:1 7.3-19. 9
C20:5 3.9-15. 2
C22:1 6.9-30. 6
C22:5 0.3-1. 3
C22:6 2.0-7. 8
C24:1 0.2-1. 3
C24:5 0.0-0. 5
15
The difference in amounts of a given fatty acid in the
commercial herring oil was not quite so great as for commercial
menhaden oil. This may have been due to the fact that there were
far less samples in the herring oil analyses than was the case
for the menhaden oil where samples were taken over an eleven year
period. If the samples in table 2-4 where the minimum amount was
0 are ignored, then the three fatty acids C22:1, Cl8:2 and C24:1
had the greatest variation (5 to 6 times). C22:1 was also one of
the fatty acids showing greatest variation in the analyses for
menhaden oil.
Both menhaden and herring oil are ones made under commercial
conditions whereby the oil was extracted by cooking and pressing.
There is just one food fish, mullet, for which the oil was
extracted by dissolving it with solvents. This species was
studied not because of any general interest in the fatty acid
content but rather because of the fact that mullet, unlike all
other fish, sometimes contains high amounts of odd carbon fatty
acids, e. g. Cl5 and Cl7. A study was carried out by taking
monthly samples for a full-year at four different locations. The
results of this study are shown in table 2-5. Not only the odd
carbon chain fatty acids but also several others showed
considerable variation. Thus, the Cl6:1 varied from 13. 4 to 29. 3
and the C22:5 from 1. 3 to 3.6.
16
Table 2-5. Range of Fatty Acid Composition of Mullet Oil.
ocean perch oil, mullet oil and herring oil. The species of
animals which were effected include rats, mice and chickens.
29
A great deal of research was carried out between 1955 and
196 6 by Peifer, working at Hormel Institute under a contract from
the Seattle laboratory of the U. S. Bureau of Commercial
Fisheries. His work went much further than merely showing that
various fish oils lowered serum cholesterol levels. For example,
he showed that some of the omega-3 fatty acids in fish oil (e. g.
C20:5 omega-3 and C22:6 omega-3) occurred at greatly increased
levels in the heart of animals fed fish oils. The levels
increased up to seven times as much as was the case when fish or
fi�h oils were not present. Work of another investigator
(Kingsbury) also has indicated that the level of omega-3 fatty
acids increased in the hearts of human individuals who had
consumed fish oil. This work of Peifer et al was carried out in
the period of the late 1950's and 1960's.
Much later d�ring the 1970's and 1980's a great deal of
research has been or currently is being carried out on omega-3
fatty acids in fish oils in the diet and its relationship to
heart disease. This originated when Danish workers noted the
fact that individuals living in remote areas of Greenland very
seldom had heart disease. Workers at the Aalborg Hospital at
Aalborg, Denmark, primarily Drs. Dyerberg and Bang, decided to
look into the cause of this situation. They sent medical
statisticians to Greenland where it was confirmed that there was
almost no heart disease in remote areas of Greenland and that the
food consumed was almost all seafood. Research was then carried
30
out in Denmark which lead to the conclusion that the high omega-3
content of the diet of Eskimos was most likely the reason that
the death rate from heart disease there was only a small fraction
of what it was in Denmark or the United States.
A similar situation was found to be the case in Japan.
Japanese fishermen and their families who live along the coast
consume a diet which contains a far larger proportion of seafood
than is the case with families living nearby in farming villages
where only very small amounts of seafood are consumed. The death
rate from heart disease in the fishing villages is far less than
that of those in the farming villages.
In very __ recent times a tremendous amount of research on the
effect of omega-3 fatty acid consumption on lowering incidence of
heart disease has been carried out. Such findings have been
established by research in Great Britain, the United States,
Canada, Germany, Australia, the Netherlands and many other
countries. The great extent of this evidence leaves no doubt
that consumption of long carbon chain omega-3 fatty acids found
in our food almost exclusively in the oil of fish can decrease
the incidence of heart disease. Much of the current work deals
with the mechanism of just how the omega-3 fatty acids bring
about this effect. It appears that the omega-3 fatty acids do
31
not carry out their effect directly but rather change the nature
of certain other compounds which are present which, in turn, slow
down the coagulation of the blood in the arteries.
Some of the current research on omega-3 fatty acids and
their effects on heart disease has been concerned with whether
higher amounts of omega-3 fatty acids in the diet might have any
detrimental effect (in addition to beneficial effect toward heart
disease). Of course slowing down of blood coagulation might be
considered a detrimental effect. However, the amount of fish in
the usual diet is not nearly as great as the amount in the diet
of the Greenland Eskimos. It has been shown that beneficial
effects toward heart disease can still result and with only a
very small increase in blood coagulation time, much less than
that of eskimos whose diet was extremely high in seafood. Other
research is continuing to see if any other detrimental effects of
omega-3 fatty acids might occur, but to date no such effect has
been established.
III. Use of fish oil for treatment of diseases other than those
of the heart
A. Arthritis
The first scientific investigation of the use of a fish oil
to treat a human disease started more than 200 years ago at the
32
Manchester Infirmary in England on the use of cod liver oil for
patients suffering from arthritis. Around 1770 this hospital had
been treating such patients by giving them doses of guaiacum. It
was customary at that time to rub the patients joints with cod
liver oil apparently based upon the idea that they were oiling
squeaky joints. Such treatment never seemed to do much good. In
1772 an outpatient with arthritis who had been coming to the
Manchester Infirmary suggested perhaps she should take the cod
liver oil internally. The physicians, while believing this would
do no good, did not object and the woman did so and quite soon
improved greatly. The hospital attendees felt that the
improvement was due not to any effect of the cod liver oil but
rather to a change in the weather which often affected arthritis.
About a year later the same woman returned to the hospital with
far worse problems with arthritis than she had previously had and
requested that she be allowed to- take cod liver oil internally.
When this was done her arthritis improved rapidly. As a result
of this case, all arthritis patients at the Manchester Infirmary
starting in 1772 were treated with cod liver oil taken
internally. Over the next decade such treatment resulted in
almost 100% improvement of the many patients involved. A paper
describing these results was read at a meeting of the Royal
Society of Physicians held in Paris in 1782 and published in 1783
in the London Medical Journal.
3 3
One problem encountered in their research was the very bad
flavor of the cod liver oil. Today, cod liver oil has a somewhat
unpleasant flavor, but it is made from fresh livers by merely
cooking the livers to release the oil. In the 1700's cod liver
oil was made by rotting the livers. This resulted in an
exceedingly vile flavor. The Manchester Infirmary got around
this problem by first changing the cod liver oil by addition of
an alkali to a soap followed by treatment with peppermint
resulting in a product not objected to by the patient.
Immediately after swallowing the cod liver oil soap, lemon juice
was taken which changed the soap back to cod liver oil. The
Manchester hospital stated that this treatment could not be used
at home because the conversion of the cod liver oil to a soap had
to be carried out in a laboratory. This situation, however,
probably accounts for the fact that this treatment never became
well known and eventually was completely forgotten.
Since 1970, however several recent investigations (although
the workers were quite unaware of the work carried out over 200
years ago) have shown that patients suffering from arthritis can
be helped by including fish or fish oil in the diet. In a recent
project published in 1987, for example, some patients were fed
MAXEPA tablets (a source of omega-3 fatty acids) while others
served as controls. The consumption of the omega-3 fatty acids
clearly showed a decrease in arthritis symptoms.
34
For another inflammation disease, nephritis involving kidney
inflammation, two different investigations using animals have
been carried out. In each investigation patients have been
benefitted by consumption of omega-3 fatty acids.
B. Multiple Sclerosis
Multiple sclerosis is a very difficult disease to
investigate as to effects of fish or fish oil in the diets. This
is because multiple sclerosis occurs only in humans, never in
animals. · With most diseases, initial research is carried out
with animals and then any favorable effects extended to humans.
Nevertheless, starting in about 1950 several attempts to
determine whether fish or fish oil in the diet could have any
beneficial effects on this disease have been investigated.
Probably the most extensive of such work has been carried out by
R. L. Swank working mostly in Oregon. He has been investigating
multiple sclerosis for over 40 years. In 1950 he and co-workers
reported that statistics on multiple sclerosis in Norway showed
that those inhabitants living inland when much meat but little
fish was consumed had a much higher incidence of multiple
sclerosis than those living on the coast where much fish was
eaten. Swank has continued such work over the years. In 1988 he
published results of a 35-year study involving more than 100
35
patients. The results indicated that there was less multiple
sclerosis among those who ate considerable fish than for those
who did not. The difference was, however, quite small.
Another investigation was carried out by two investigators
in Illinois, Bernsohn and Stephanides. Most of their work dealt
with comparing statistics on incidence of multiple sclerosis in
different countries throughout the world. They had a hypothesis
that even small traces of omega-3 fatty acids in the diet were
sufficient to prevent the disease. They believed that
individuals who developed multiple sclerosis probably had faulty
enzyme systems for converting fatty acids to traces of omega-3's.
They attempted to carry out experiments on two islands off the
coast of Scotland. One island was inhabited by fishermen and
there was much fish in their diet. The other island was occupied
by farmers who ate little fish. There were more cases of
multiple sclerosis among families living on the island where
farming was the occupation. However, because the population of
these islands was small, the number of cases of multiple
sclerosis were so few that no statistically significant
differences occurred; consequently the results of these tests
were never published.
Recently (1989) a report of an investigation on treatment of
multiple sclerosis with omega-3 fatty acids was published. This
investigation involved 292 individuals half of which consumed
36
omega-3 fatty acids, the other none. As was the case in the
investigation of Bernsohm and Stepanides made about thirty years
earlier there was no difference at the 95% confidence level.
However, after two years only 65 patients not consuming omega-3
fatty acids were improved as compared to 79 for those ingesting
the omega-3 fatty acids. It would appear that although omega-3
fatty acids in the diet may have some small beneficial effect,
the improvement for multiple sclerosis is not of any great extent
if any at all.
C. Cancer
Work on looking into the effect on cancer of fish oil omega-
3 fatty acids began in 196 7. Since then, several investigations
most using animals but some using humans have been conducted
dealing with both breast cancer and colon cancer. Although the
extent of such research has not been nearly as great as that for
heart disease, there has been considerably more work on breast
and colon cancer than on any disease other than that carried out
dealing with the heart. It would appear from presently completed
research on breast and colon cancer that probably the ingestion
of fish oi l omega-3 fatty acids has a beneficial effect.
Considerable more such research is currently underway.
37
D. Other Diseases
There is no evidence that omega-3 fatty acids lower the
incidence of strokes, but one investigation using animals would
indicate that perhaps the degree of damage done to body tissues
as a result of a stroke may be less when omega-3 fatty acids have
been ingested. Several investigations indicate that certain skin
diseases such as psoriasis may be alleviated by consumption of
omega-3 fatty acids. There is some evidence that high blood
pressure can be diminished by consumption of fish oil omega-3
fatty acids. Recently, some work has been carried out on effects
of consumption of fish oil omega-3 fatty acids upon certain
problems with the eye and also with the brain. Work in these
areas is fairly new but considerable research is now underway
along such lines.
VI. Current Extent of Research on Omega-3 Fatty Acids and
Disease
Research on effects of omega-3 fatty acids and disease began
in the 1970's at a slow pace. Over the years, there has been a
rapid expansion in the number of such investigations. The extent
of the present level of such work can be estimated by the number
of investigations reported at the International Conference on the
Health Effects of Omega-3 Polyunsaturated Fatty Acids in Seafoods
held in March 1990 in Washington, D. C.
38
Table 3-1. Objectives of Investigations Underway in 1990 on Omega-3 Effects on Disease as Measured by Topics Reported at March 1990 International Conference on Health Effects of Omega-3 Fatty Acids in Seafoods.
Topic Number of Investigations
Omega-3 and Heart Disease 35 Omega-3 Effects on non-Human Subjects 34 Omega-3 Alteration Mechanisms 18 Omega-3 and Cancer 10 General and Miscellaneous 10 Omega-3 and Blood Pressure 6 Omega-3 and Food Consumption 6 Omega-3 Effects on Eye and Brain 5 Omega-3 Effects on Serum Cholesterol Levels 5 Omega-3 Consumption Effects on the Elderly 4 Omega-3 Consumption Effects on Infants 4 Omega-3 Concentrates Preparation 3 Omega-3 Effects on Immune Response 3 Omega-3 Effects on Pregnancies 3 Omega-3 Effects on Skin Diseases 3 Omega-3 Effects on Miscellaneous Inflammation Diseases 2 Omega-3 and Microalgae 2 Omega-3 Effects on Arthritis 1 Omega-3 Effects on Diabetes 1 Omega-3 and Ether Linked Phospholipids 1 Omega-3 Effects on Malaria 1
At this meeting both full research papers and poster sessions were pre�ented as follows:
Full research papers read 3 3 Number of poster sessions presented �
Total 120
This total of 120 investigations are of course, not all of
those underway in 1990. Undoubtedly there were a great many more
which were being carried out but from which no individual was
present at the meeting. It would seem likely that the total
number of such investigations underway in 1990 exceeds 200.
39
Table 3-1 lists the topics discussed at this March 1990
meeting of the International Conference on Health Effects of
Omega-3 Fatty Acids in Seafoods. The topics are arranged in
order of the number of investigations for each topic. The topic,
omega-3 and heart disease still leads the list with 35
investigations. The second largest number of investigations (3 4)
is effects of omega-3 fatty acids on non-human subjects. While
many of these are tests using mice or rats, there are a great
many also on effects upon, for example, rabbits, fish, monkeys,
pigs and birds. A topic which is increasing in number of
investigations deals with_ study of the various mechanisms whereby
during ingestion of omega-3 fatty acids such acids are altered to
produce other substances. There were 18 such investigations
listed. The disease other than heart disease for which the
greatest number of investigations were listed is cancer. Most of
these dealt with breast cancer; there were a few on colon cancer.
Another topic for which there is increasing interest is the
effects of omega-3 fatty acids on the brain and the eye (5
investigations).
In this Chapter 3 there has been discussed effects of
ordinary fish oil upon diseases of humans. This fish oil is
largely in the form of triglycerides or phospholipids. There are
other forms of fish lipids such as, for example glyceryl ethers
which will be discussed in Chapter 6.
40
Chapter 4. Deterioration of Fish Oils and its Effects
I. Introduction
The oils in fish are extremely susceptible to deterioration
both while still in the flesh of fish but especially after they
have been extracted. The deterioration in most cases is caused
by the much greater liability for oxygen from the air to combine
with the fatty acids of which the oil is composed. For the most
part fatty acids in fish occur as triglycerides wherein three
fatty acids occur thus:
C Fatty acid #1 I
Fatty acid #2 I C Fatty acid # 3
The three fatty acids are most frequently each different ones.
This results when the fish oils oxidize in the development of
disagreeable flavors described as fishy and rancid. As mentioned
in Chapter 2, the fatty acids of fish oil contain up to six
double bonds. The oxygen from air attacks these double bonds
yielding components having disagreeable flavors usually described
as fishy or rancid. Whereas the oils in other foods such as
meats, poultry or vegetables contain in each molecule usually
only 2 or at the most 3 such double bonds, in fish oil their
fatty acids can have up to six such double bonds making the fish
oils very highly polyunsaturated and therefore much more highly
subject to development of disagreeable rancid flavors.
41
The rate at which oxygen from the air combines with the
polyunsaturates of fish oil can be reduced by addition of
antioxidants. However, this reduction is not nearly as great as
is the case with antioxidants applied to other oils. With fish
oils the only way to completely stop the oxidation is to store
the fish or its oil in the complete absence of air. For extended
storage of fish oils it is necessary to bubble an inert gas such
as nitrogen through the fish oil in order to eliminate air
dissolved in the oil.
In addition to any antioxidants that might be added to help
stabilize a fish oil, all fish oils contain varying amounts of
naturally occurring antioxidants. The most frequently occurring
such natural antioxidant is tocopherol (vitamin E). The amount
which occurs naturally in the oils from different species varies
to a considerable extent. See Table 4-1.
4 2
Table 4-1 . Tocopherol (Vitamin E) Content in the Oil of Flesh of Several Species of Fish
Species
Sardine Menhaden Tuna Herring Whale Sablefish
Tocopherol Content (micrograms per gram of oil)
4 0
70 160 140 220
6 30
The stability against oxidation follows quite closely the
increase in tocopherol content; thus sardine oil is one of the
most unstable fish oils becoming rancid very rapidly when exposed
to air . On, the other hand, sablefish, although containing
considerable oil in the flesh, is one of the most stable species
known .
I I. Flavors and Odors During Oxidation of Fish Oils
A . Flavor and Odor of Pure Unoxidized Oil
First to be considered is the relationship between flavor
and odor of fish oil . In the case of extracted fish oil, while
the flavor and the odor are closely related ordinarily the flavor
is much more easy to assess than that of the odor . The odor is
in most cases quite faint for completely unoxidized oil . The
flavor, on the other hand, often can be assessed in two stages .
First there is the initial flavor which is noted at once when the
43
oil is first inserted in the mouth. After a short time, the
flavor often changes resulting in what is known as the
aftertaste. This aftertaste is frequently more pronounced than
the initial flavor.
The flavor of unoxidized fish oil ordinarily varies from
that made from different species of fish. That is to say many
species of fish have odors and flavors in their fresh unaltered
stage which is characteristic for that species. For example
salmon oils have such a characteristic flavor or odor. In most
cases we do not as yet know just which chemical substances in the
salmon oil, for example, are recognized as resulting in the
flavor we associate with salmon. In a similar way the flavors of
the oil in fresh fish of many other species such as mackerel,
herring, and lake trout have flavors which are associated with
the various species in their fresh state.
B. Flavors and Odors of Oxidized Oils
One of the earliest types of odors and flavors which develop
upon oxidation of the oils in fish are a type designated as
"fishy" . These so-called fishy flavors and odors are of several
varieties and some of them are shown in Table 4-2.
4 4
Table 4-2. Flavors and Odors Described as "Fishy"
Description Burnt
Freshwater Fish
Green
Iodine-Like
Pure Oxidation Types
Sweet: An intense sweet odor generally considered quite unobjectable
Predominance as flavors (F) or odors (0)
Principal Source Fish oil and fish meals
Freshly caught freshwater fish in skin or flesh
Fish oils
Certain species such as sole especially after they have been feeding on some special particular types of food
Fish oils
Well . iced fish
Primary Secondary 0 F
0
F
F
F
0
F
0
0
0
F
The term fishy odor is not exclusively used for odors in
fish. For example butter may be described as fishy. The flavor
described in this way is either identical or nearly so to that
described in fish as fishy. Other foods which may at times be
said to have fishy odors include poultry products. This occurs
often when the poultry are fed fish at excessively high levels.
Often the poultry are fed fish until shortly before slaughter and
then fish are eliminated from the diet which diminishes the
fishiness.
45
On the other hand the term "fishy" may be used to describe
an entirely different type of flavor. Thus, in one instance, a
panel carrying out storage tests on meat described a pork sample
as fishy. When this same sample was examined by a panel working
with fish, the pork sample was described not at all as fishy, but
rather as having a mutton flavor.
The chemical compounds responsible for the fishy odors are
quite numerous. For example in one study of crude menhaden oil
55 different chemical compounds were found. Some of these
together with the description of their various odors are shown in
Table 4-3.
4 6
Table 4-3. Some Compounds with their Quantity and Odor Found in a Crude Menhaden Oil
Number '!
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17
18 19 20 21 22 23 24 25 26
Odor Characteristic
painty cut grass, green greasy green, musty sickly sweet, cooling waxy green, grassy sharp green, oily pesticide-like pesticide-like citrus, fatty, orange sharp, green, greasy pesticide-like musty with citrus topnote fatty floral musty, waxy, floral irritative, vinegar like vegetable green